The present invention relates to a novel copolymer resin of maleimide and alicylic olefin-based monomers, a photoresist containing the copolymer resin and to a process for the preparation thereof. More specifically, the present invention relates to a novel copolymer resin of maleimide and alicyclic olefin-based monomers useful for a photoresist usable for lithography process using ultra-short wavelength light source such as KrF(.lambda.=248 .mu.m), ArF(.lambda.=193 .mu.m), X-ray, ion beam, E-beam and EUV (Extreme Ultra Violet) which is a potentially applicable technology to the fabrication of a fine circuit of a highly integrated semiconductor device. The present invention also relates to a photoresist formulated by using the above copolymer resin and to a process for the preparation thereof.
To achieve a high sensitivity in the integration process of semiconductor device, a chemical amplification photoresist for DUV (deep ultra violet) light source has been prevailing. The chemical amplification photoresist consists generally of a photoacid generator and a matrix polymer having a chemical structure which sensitively reacts with acid.
The mechanism of such a chemical amplification photoresist is as follows. The photoresist is exposed through a mask to a ultra violet light source. An acid is generated by the action of the photoacid generator and then, reacts with the main or side chain of the matrix polymer. This reaction surprisingly increases the solubility of the copolymer in a developing solution by changing the structure of the polymer, e.g., by decomposing it, cross-linking it or changing its polarity. Therefore, at exposed regions the copolymer is dissolved in the developing solution, whereas at un-exposed regions the copolymer has no change in its original structure and remains undissolved in the developing solution, so that the shape of the mask may leave as a positive image on a substrate. In the above lithographic process, the resolution depends on the wavelength of light source. Accordingly, the smaller the wavelength of light source, the finer the pattern formulated.
In general, a photoresist is required to have light transparency, etching resistance, thermal resistance and adhesion. In addition, the photoresist used for ArF photoresist must be developed in 2.38% TMAH aqueous solution. However, it would be difficult to prepare a copolymer resin which satisfies all the properties of photoresist. For instance, the copolymer resin having polyacrylate-based main chain structure can easily be synthesized, but there are problems in an etching resistance and in a development process. The etching resistance can be improved by introducing alicyclic unit into main chain of the copolymer resin, but it is difficult to substitute the entire main chain by alicyclic unit.
In an attempt to solve the above problems, Bell Lab. has proposed a copolymer resin of the following formula (1), having a main chain structure substituted by norbornene, acrylate and maleic anhydride. ##STR1##
In this copolymer resin, maleic anhydride, A-moiety of the formula (1), used for the polymerization of an alicyclic olefin group does not absorb a light of 193 nm in wavelength and is the only material capable of polymerizing with the alicyclic unit, i.e., norbornene. However, the maleic anhydride, upon non-exposure, is easily dissolved in 2.38% TMAH aqueous solution and occurs top-loss phenomenon in which the top of the photoresist pattern becomes round. Therefore, this copolymer resin is not suitable to the resin for KrF or ArF lithography.
Accordingly, to inhibit dissolution, it is necessary to increase the ratio of t-butyl substituent, y-moiety of the formula (1). In this instance, the ratio of carboxylate substituent, Z-moiety of the formula (1), which increases the adhesion is reduced and thus, the photoresist is departed from the wafer upon actual patterning and the pattern formulation is not possible.
In addition, upon post exposure delay which does not conduct a baking immediately after exposure, the bottom of the pattern is shorter than the top of the pattern. That is, T-top phenomenon occurs and thus the pattern formulation itself is not possible. Also, since maleic anhydride reacts with hydroxyl group (--OH) which increases adhesion, there is a possibility of giving an influence on a shelf life of photoresist.
Accordingly, Bell Labs. has attempted to solve such disadvantages by introducing as a bi-component dissolution inhibitor an alternating copolymer of cycloolefin and maleic anhydride. However, since this method must use a dissolution inhibitor in the excessive amount, about 30% by weight of the copolymer resin, a reproductivity of the resin is low and the cost is increased. Accordingly, this resin also is not suitable as a photoresist resin.